There is a type of image forming apparatus that includes a fixing device of a type utilizing induction heating performed by one induction heating coil (coil) to heat up a fixing roller (heating roller). In order to prevent the heating roller from being abnormally overheated, like at the time of a failure etc. of the image forming apparatus, an overheating prevention valve (overheating prevention element), such as a thermostat, a thermal fuse, etc., is provided near the surface of the heating roller. In a conventional induction-heating type of fixing device whose heating roller contains one coil therein, the single coil is provided to a central portion (a part of the surface of the heating roller substantially at the midpoint in the longitudinal direction of the heating roller) where the surface temperature becomes the highest when an abnormal condition occurs.
There is another type of image forming apparatus including a fixing device of a type containing a plurality of lamps serving as heat sources inside its heating roller. In such an image forming apparatus, the highest-temperature portions of the heating roller are the portions on the surface corresponding to the positions of the lamps. Accordingly, one overheating prevention valve is provided to one of these portions, which can be monitored.
In a fixing device of the type containing only one coil in its heating roller, sometimes it happens that there is a considerable difference in surface temperature between the central portion and the end portions of the heating roller. That is, in the warming-up mode (when no paper is fed), the temperature rise rate of the central portion of the heating roller is rather high. On the other hand, in the printing mode (when paper is fed), since paper being fed removes heat from the heating roller, the temperature rise rate of the end portions, which are the areas not associated with the printing operation since they do not contact the paper, becomes relatively high. Accordingly, in order to prevent the rise in temperature at the end portions of the heating roller in the printing mode, air cooling control, etc., using a fan (end portion cooling fan) or the like is carried out. Of course, the end portion cooling fan is not activated in the warming-up mode.
As described above, in the warming-up mode, the temperature at the central portion of the heating roller rises. Accordingly, it is natural that if the oscillation of coil continues after the temperature of the heating roller reaches the fixing temperature due to the occurrence of an abnormal condition of the thermistor, etc., and the heating roller is continuously heated, the surface temperature of the central portion becomes highest. Therefore, conventionally, one overheating prevention valve was provided to the central portion.
On the other hand, in the printing mode, the coil is alternately oscillated and stopped in order to maintain the temperature of the heating roller in a fixable range. The paper being fed removes heat from the portion of the surface of the heating roller contacting the paper, resulting in that the temperature of the end portions that do not contact the paper becomes relatively high. However, because of the air-cooling control performed by the end portion cooling fan, the temperatures of the end portions and the central portion become substantially the same. In such a state, if an abnormal condition occurs to the ON/OFF control of the coil, resulting in that the heating roller is continuously heated, i.e., the coil is continuously oscillated without stopping, the temperature of the central portion becomes the highest. From this standpoint, it is sufficient that one overheating prevention valve is provided at the central portion.
In such a fixing device, coils used exclusively for the end portions may be newly provided inside the heating roller to achieve heating by a plurality of coils. In such a case, overheating prevention devices may be provided in a one-to-one relationship with the coils.
With such an arrangement, the overheating prevention devices are expected to monitor the highest temperature portions of the heating roller. However, sometimes it may happen that the portions between the coils have the highest temperature. In such a case, the above arrangement is no longer the optimum coil arrangement. The reason is as follows. The overheating prevention devices are provided for the purpose of preventing overheating of the heating roller at the time of the occurrence of an abnormal condition; for this purpose, the overheating prevention devices must surely monitor the highest temperature portions of the heating roller in every case; however, in the above arrangement, no overheating prevention valve is provided at the most overheated portions of the heating roller.